[Go] Generate test files

Created by: gcatanese

@antihax (2017/11) @grokify (2018/07) @kemokemo (2018/09) @jirikuncar (2021/01) @ph4r5h4d (2021/04)

Updated Go codegen to generate the test files as well, adding a new api_test.mustache. See #13559 (closed).

I have provided a basic implementation of a test, and added the skip option by default (I think it is very helpful to have a basic test implementation to start with, which can be extended and enabled by developer)

How to validate

Generate Go client and verify the test files have been created

java -jar /openapi-generator/modules/openapi-generator-cli/target/openapi-generator-cli.jar  generate \
-i openapi.yaml -g go  -p packageName=Management -o src/folder

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (6.1.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.